The present invention relates to a method of welding a disposable diaper along laterally opposite side edges at its waist level.
One example of a pant-type of disposable diapers is disclosed in Japanese patent application Disclosure No. 5-15551. In that reference, a diaper comprises top- and backsheets and a liquid-absorbent core sandwiched therebetween. The diaper is folded along a transverse center line passing through a crotch zone so that front and rear bodies are positioned one upon another. The top- and backsheets are ultrasonically welded together along laterally opposite side edges at its waist level to obtain a pant-type configuration. The top- and backsheets may be formed, for example, by nonwoven fabric of thermoplastic synthetic resin which can be ultrasonically heated.
Nonwoven fabric employed in the technique disclosed in said Disclosure as material for the top- and backsheets may be made from fibers of the same type as mentioned above and heated until the nonwoven fabric exhibits an appropriate viscosity so that a weld zone of the top- and backsheets presents a smoothly finished appearance. However, if the topsheet formed of nonwoven fabric of polypropylene fiber having a relatively high melting point is combined with the backsheet formed of polyethylene sheet or film having a relatively low melting point, the polyethylene sheet will melt earlier than the polypropylene fiber when the diaper is folded and subjected to the ultrasonic treatment. In consequence, the polyethylene sheet may melt to an excessively low viscosity and stick to a horn pressed against this sheet during the ultrasonic treatment. The polyethylene sheet which has stuck to the horn will, once cooled, result in a rough, itchy and uncomfortable surface and spoil the aesthetical appearance of the diaper. If the polyethylene sheet remains stuck to the horn, no accurate clearance can be assured between the horn and an anvil during the next cycle of the ultrasonic treatment, thus making the continuous treatment difficult.
Accordingly, it is a principal object of the invention to solve the aforementioned problem by placing a covering sheet having a melting point higher than a melting point of a backsheet upon an outer surface of the backsheet and bringing it in contact with the pressing means such as a horn for an ultrasonic treatment.